1. Field of the Invention
The present invention relates to a method for forming crank shafts and a method for producing crank shafts, which make it possible to reduce the processing cost and shorten the processing time by decreasing the number of processing steps.
2. Description of the Related Art
Conventionally, the crank shaft to be used as an automobile part has been produced such that a steel material having a columnar configuration, which has been cut into a predetermined length, is subjected to the forging, followed by the cutting processing step.
The steps for producing the crank shaft concerning the conventional technique will be briefly explained below, taking notice of the processing performed for a shaft and a crank pin (referred to as "shaft section") which constitute the crank shaft.
At first, a steel material, which has been heated to have a red-heated appearance, is subjected to the buster processing and the coarse (preliminary) processing, followed by performing the finishing processing. In the finishing processing, as shown in FIG. 7, a first intermediate formed product 7a is obtained by means of the forging processing based on the use of an upper die 1a and a lower die 1b. The upper die 1a and the lower die 1b comprise recesses 3a, 3b for forming a space having a substantially columnar configuration with a diameter .phi.D therebetween when they make nearest approach to one another during the processing, and planar sections 5a, 5b disposed on both sides of the recesses 3a, 3b which are opposed to one another while leaving a spacing distance La therebetween.
The first intermediate formed product 7a, which is obtained by the finishing processing, has a columnar section 9 having the diameter .phi.D corresponding to the recesses 3a, 3b and a burr 11 corresponding to the planar sections 5a, 5b. The thickness of the burr 11 is La. In this case, the diameter .phi.D is a value obtained by adding a predetermined machining allowance "a" to a diameter .phi.d of the shaft section of the crank shaft. That is, there is given .phi.D=.phi.d+2.times.a. The reason why the first intermediate formed product 7a is specially formed is that it is intended not to allow the inside of the formed product to be sparse by applying the processing to the steel material which is sufficient to obtain the fluidity of the lump during the forming process.
Subsequently, the trimming processing is performed. In the trimming processing, as shown in FIG. 8, the columnar section 9 is stamped out from the first intermediate formed product 7a placed on a die 13a by using a punch 13b to obtain a second intermediate formed product 7b. In this case, the width dimension of a hole 15 provided for the die 13a has a value obtained by adding a predetermined machining allowance "b" to the diameter .phi.D, i.e., D+2.times.b. As shown in FIG. 9, the second intermediate formed product 7b, which is obtained by the trimming processing, has a shape including a burr 11 having a burr height (with the value "b" as well) which remains on the columnar section 9 corresponding to the machining allowance "b".
Subsequently, the second intermediate formed product 7b is subjected to the restriking to correct the bending of the shaft generated when the respective processing steps are performed as described above.
Subsequently, the cutting processing (lace processing) is performed to obtain a third intermediate formed product (not shown) from which a part of the machining allowance "a" and the burr height "b" have been removed. Subsequently, the soft nitriding treatment (tufftride treatment) or the local high frequency heat treatment is applied thereto, and then the finishing polishing processing is performed. Thus, the remaining portion of the machining allowance "a" is removed to obtain the crank shaft including the shaft section which has the diameter .phi.d.
In the finishing processing described above, the upper die 1a and the lower die 1b do not make abutment with each other. That is, the gap of the spacing distance La exists between the upper die 1a and the lower die 1b. For this reason, it is feared that the thickness of the first intermediate formed product 7a may involve any dispersion. Further, it is feared that any deviation may occur between the portion (upper portion) of the first intermediate formed product 7a corresponding to the upper die 1a and the portion (lower portion) of the first intermediate formed product 7a corresponding to the lower die 1b, on the basis of the positional deviation between the upper die 1a and the lower die 1b.
When the finishing processing is performed, the steel material is heated to have a red-heated appearance. Further, the processing often brings about an amount of deformation of the steel material. Furthermore, the steel material has a heavy weight, and it has a columnar configuration which is long in the axial direction. Therefore, the steel material undesirably bites into the upper die 1a and the lower die 1b during the finishing processing. A large force is required to perform the operation (ejecting operation) for removing the steel material (first intermediate formed product 7a) from the upper die 1a and the lower die 1b. As a result, any deformation including, for example, bending of the axis occurs due to the force which is applied to the first intermediate formed product 7a during the ejecting operation. That is, there is a possibility that the axis may be bent at the columnar section 9 of the first intermediate formed product 7a. Further, a large shearing force is applied to the columnar section 9 when the trimming processing is performed. Therefore, it is feared that the axis of the columnar section 9 may be also bent by the shearing force.
Even when the restriking is effected for the second intermediate formed product 7b in which the deformation has occurred as described above, for example, any springback takes place. For this reason, it is impossible to completely correct the deformation. Therefore, in order to obtain the third intermediate formed product having the columnar section 9 in which the cross section is a perfect circle and no bending occurs in the axis, the following procedure is required. That is, it is necessary that the machining allowance "a", which is provided for the columnar section 9, is previously set to have a large value while considering the deviation and the bending of the axis generated in the columnar section 9, and the machining allowance "a" is removed by means of the cutting processing.
Further, the burr 11 having the predetermined burr height "b" remains in the second intermediate formed product 7b. Therefore, it is necessary that the cutting processing is performed to remove the burr 11 as well.
As described above, in the process for producing the crank shaft concerning the conventional technique, it is necessary that the machining allowance "a" and the burr 11, which have been provided to be large, are removed by means of the cutting processing. Therefore, inconveniences arise in that the production cost and the processing time for the crank shaft are increased.